The present invention relates to cloning vectors and methods for producing a library of DNA molecules capable of expressing a fusion polypeptide on the surface of a filamentous phage particle.
Filamentous bacteriophages are a group of related viruses that infect bacteria. They are termed filamentous because they are long and thin particles comprised of an elongated capsule that envelopes the deoxyribonucleic acid (DNA) that forms the bacteriophage genome. The F pili filamentous bacteriophage (Ff phage) infect only gram-negative bacteria by specifically adsorbing to the tip of F pili, and include fd, f1 and M13.
The mature capsule of Ff phage is comprised of a coat of five phage-encoded gene products: cpVIII, the major coat protein product of gene VIII that forms the bulk of the capsule; and four minor coat proteins, cpIII and cpIV at one end of the capsule and cpVII and cpIX at the other end of the capsule. The length of the capsule is formed by 2500 to 3000 copies of cpVIII in an ordered helix array that forms the characteristic filament structure. About five copies each of the minor coat proteins are present at the ends of the capsule. The gene III-encoded protein (cpIII) is typically present in 4 to 6 copies at one end of the capsule and serves as the receptor for binding of the phage to its bacterial host in the initial phase of infection. For detailed reviews of Ff phage structure, see Rasched et al., Microbiol. Rev., 50:401-427 (1986); and Model et al., in xe2x80x9cThe Bacteriophages, Volume 2xe2x80x9d, R. Calendar, Ed., Plenum Press, pp. 375-456 (1988).
The assembly of a Ff phage particle involves highly complex mechanics. No phage particles are assembled within a host cell; rather, they are assembled during extrusion of the viral genome through the host cell""s membrane. Prior to extrusion, the major coat protein cpVIII and the minor coat protein cpIII are synthesized and transported to the host cell""s membrane. Both cpVIII and cpIII are anchored in the host cell membrane prior to their incorporation into the mature particle. In addition, the viral genome is produced and coated with cpV protein. During the extrusion process, cpV-coated genomic DNA is stripped of the cpV coat and simultaneously re-coated with the mature coat proteins. The assembly mechanisms that control transferral of these proteins from the membrane to the particle is not presently known.
Both cpIII and cpVIII proteins include two domains that provide signals for assembly of the mature phage particle. The first domain is a secretion signal that directs the newly synthesized protein to the host cell membrane. The secretion signal is located at the amino terminus of the polypeptide and targets the polypeptide at least to the cell membrane. The second domain is a membrane anchor domain that provides signals for association with the host cell membrane and for association with the phage particle during assembly. This second signal for both cpVIII and cpIII comprises at least a hydrophobic region for spanning the membrane.
cpVIII has been extensively studied as a model membrane protein because it can integrate into lipid bilayers such as the cell membrane in an asymmetric orientation with the acidic amino terminus toward the outside and the basic carboxy terminus toward the inside of the membrane. The mature protein is about 50 amino acid residues in length of which 11 residues provide the carboxy terminus, 19 residues provide the hydrophobic transmembrane region, and the remaining residues comprise the amino terminus. Considerable research has been done on the secretion signal region of cpVIII to advance the study of membrane protein synthesis and targeting to membranes. However, little is known about the changes that are tolerated in the structure of the cpVIII membrane anchor region that would allow for assembly of phage particles.
Manipulation of the sequence of cpIII shows that the C-terminal 23 amino acid residue stretch of hydrophobic amino acids normally responsible for a membrane anchor function can be altered in a variety of ways and retain the capacity to associate with membranes. However, those anchor-modified cpIII proteins lost their ability to genetically complement gene III mutants indicating that the requirements of a membrane anchor for functional assembly have not been elucidated.
Ff phage-based expression vectors have been described in which the entire cpIII amino acid residue sequence was modified by insertion of short polypeptide xe2x80x9cepitopesxe2x80x9d [Parmely et al., Gene, 73:305-318 (1988); and Cwirla et al., Proc. Natl. Acad. Sci. USA, 87:6378-6382 (1990)] or an amino acid residue sequence defining a single chain antibody domain. McCafferty et al., Science, 348:552-554 (1990). These hybrid proteins were synthesized and assembled onto phage particles in amounts of about 5 copies per particle, a density at which normal cpIII is usually found. However, these expressed fusion proteins include the entire cpIII amino acid residue sequence and do not suggest fusion proteins that utilize only the carboxy terminal membrane anchor domain of cpIII.
In addition, no expression system has been described in which a phage coat protein has been engineered to allow assembly of a heterodimeric molecule that is functional and capable of incorporation into the coat of a phage particle.
A new surface-integration technology has been discovered for expressing a heterodimeric recombinant gene product on the surface of a filamentous phage containing the recombinant gene. The invention uses a filamentous phage coat protein membrane anchor domain as a means for linking gene-product and gene during the assembly stage of filamentous phage replication.
That is, during filamentous phage replication, coat proteins assemble into a matrix which encapsulates the phage genome. It has now been discovered that (1) phage assembly is not disrupted when recombinant filamentous phage coat proteins are present, (2) recombinant filamentous phage coat proteins can be integrated into the assembling matrix, and (3) integration into the matrix can be directed to occur in a surface-accessible orientation.
The present invention can be advantageously applied to the production of heterodimeric receptors of predetermined specificity, i.e., it can be used to produce antibodies, T-cell receptors and the like that bind a preselected ligand.
Thus, the present invention provides for linking the functions of heterodimeric receptor recognition and filamentous phage replication in a method for isolating a heterodimeric receptor and the gene that encodes receptor. The method produces a filamentous phage comprised of a matrix of gene VIII-encoded proteins that encapsulate a recombinant genome. The recombinant genome contains genes encoding the heterodimeric receptor polypeptides. The heterodimeric receptor is surface-integrated into the encapsulating matrix via a filamentous phage coat protein""s membrane anchor domain that is fused by a peptide bond during translation to one of the heterodimeric receptor polypeptides. The heterodimeric receptor polypeptides and the genes which encode the polypeptides are physically linked during the assembly stage of the phage replication cycle. Specifically binding the receptor-coated phage to a solid-support advantageously provides a means for isolating a recombinant genome that encodes a desired heterodimeric receptor from a diverse library of recombinant genomes.
In one embodiment, the present invention contemplates an antibody molecule comprising heavy- and light-chain polypeptides, said heavy-chain polypeptide comprising a VH-domain flanked by an amino-terminal prokaryotic secretion signal domain and a carboxy-terminal filamentous phage membrane anchor domain, said light chain polypeptide comprising a VL-domain fused to an amino-terminal prokaryotic secretion signal domain.
In another embodiment, the present invention contemplates a vector for expressing a fusion polypeptide, said vector comprising upstream and downstream translatable DNA sequences operatively linked via a sequence of nucleotides adapted for directional ligation of an insert DNA, said upstream sequence encoding a prokaryotic secretion signal, said downstream sequence encoding a filamentous phage membrane anchor, said translatable DNA sequences operatively linked to a set of DNA expression signals for expression of said translatable DNA sequences as portions of said fusion polypeptide.